Method of treating earth formations



June 2, 1959 Filed March 19, 1957 W. H. CLARK METHOD OF TREATING EARTHFORMATIONS .Sand

lender 54 .,5.. grammo n H TTORNEVS United States Patent O 2,888,988NmTI-IOD F TREATING EARTH FORMATIONS William H. Clark, Tyler, Tex.,assigner to The Dow Chemical Company, Midland, Mich., a corporation ofDelaware Application March 19, 1957, Serial No. 647,062 i 11 Claims.(Cl. 166--15) The invention relates to methods of treating fluid-bearingearth formations into which a well is drilled so as to obtain from ordispose thereinto a fluid with greater facility. The invention moreparticularly concerns the treatment of a petroleum fluid-bearingformation into which a well is drilled so as to facilitate ow of oil orgas to the well from the formation.

One of the methods currently in use for increasing the fluidpermeability and the productivity of earth formations, particularlythose in which oil and gas wells are drilled, consists `in making aninjection of a particulated solid material, such as sand, into theproducing formation to form masses therein of high fluid permeability.In making such injections, the sand, or other particulated solidmaterial, is suspended in a uid vehicle or carrying agent, such as anoil, for example. The suspension is injected into the producingformation by pumpingV it through the well from a supply maintained atthe head of the well. But attempts to make such injections are notalways successful because the particulated solid material tends tofilter out of the vehicle at the face of the earth formation in the wellhole instead of passing into the earth formation with the fluid vehicle.particulated material filters out, such a condition is commonly referredto as a screen-out.

Accordingly, .it is an object of the invention to provide an improvedmethod of injecting a particulated solid material into an earthformation penetrated by a well bore whereby the risk of the particulatedsolid material filtering out of the vehicle in the well hole ratherthanentering the earth formation is reduced, if not eliminated, and theinjection can be made usually without encountering excessive injectionpressures.

Another object of this invention is to provide a method of changing theconcentration of particulated materials with respect to the carryingagent at any time at the `point along the well bore where the injectedmaterials enter the earth formation.

When the j In carrying out the invention, which may be practiced in awell provided with the usual casing and a tubing string extending fromthe surface to at least near the formation to be treated, `the mixtureor slurry of particulated material and carrying fluid is pumped into thewell and thence into the formation. As injection of the mixture into theformation proceeds a diluent fluid is also injected linto the formationwhen the conditions of injection of the mixture indicate that a screenout is beginning of particulated material on the face of the formation.

The diluent fluid so injected dilutes the mixture of particulatedmaterial and carrying agent or fluid about to enter the formation andthereby facilitates its passage thereinto, reducing or eliminating thescreen out.

The invention, as well as additional objects and advantages thereof,will best be understood when the following detailed description is readin connection with the yaccompanying drawing in which the singlegureshows,

in diagrammatical form, a well which is coupled to apparatus. used in,practicing this invention. r

Referring to the drawing, a well bore 10 extends from the surface 12 ofthe earth through an earth formation or formations 14 and into orthrough an oil bearing formation 16, commonly called a pay zone. .Astring of casing 18 extends from the surface 12 of the earth to thebottom 20 of the well bore. The space between the casing 18 and the wallof the well bore 10 is filled with cement 22. In the drawing the cementis shown extending from the bottom 20 of the well bore 1.0 to thesurface 12 of the earth, but in practice the cement sometimes extendsonly part Way between the bottom 20 of the IWell bore 10 and the surface12.

The Iupper end of the casing 18 is capped by a casing head 24 ofsuitable design through which extends a string of tubing 26 (or casingof smaller diameter than tine diameter of the casing 18). The lower end28 of the tubing 26 is disposed near to the perforations 30 in the wellbore wall (and casing 18) which communicate with the oil bearingformation or pay zone 16. The phrase near to as applied to thedisposition of the lower part of the tubing from which diluent fluid mayflow (the end of the tubing could be closed and the tubing walls beperforated) means at, slightly above, or slightly below the casingperforations 30.

A source 32 of diluent iiuid, such as lease oil or water, for example,is coupled to the tubing 26 through the line 34, valve 36, and pump 38.

The casing 18 is coupled through a line 40, valve 42 and pump or pumps44 to a blender 46 in which particulated materials and carrying agentare mixed prior to being pumped into the annulus between the casing 18land tubing 26. A hopper 48 adapted to contain particulated materials,such as sand or crushed walnut shells, by way of example, is coupled tothe blender 46 through `a suitable flow control means 50. (It is, ofcourse, recogm'zed that the particulated material may be manuallyapplied to the blender.) A reservoir or tank 52 of carrying agent oriiuid such as lease oil, for example, is coupled to tlhe Iblender 46through the line S4, valve 56 and pump 58.

In operation the annulus between the tubing 26 and the casing 18, aswell as the tubing, are often pumped full of clear carrying agent fluid68 at the beginning of the formation fracturing treatment, the gaspresent in the casing and tubing being vented from the casing head 24through the line 60 with the valve 62 open. When the tubing 26 andannulus are filled with carrying agent or other suitable liquid, thevalve 62 is closed. Usually more clear carrying agent is then pumpedinto the annulus to increase the vhydrostatic pressure and break downthe earth formation (pay zone) adjacent to the perforations beforeparticulated materials are blended into the carrying agent. The abovedescribed precautionary step of breaking down the earth formation beforeinjecting into the annulus any blend of carrying agent and parjected andthe formation 16 did-nt break down and permit the fracturing slurry toenter the formation. Once a screenout occurs, considerable time andeffort isl -required `to ilush from the well bore the screened-outparticulated material before the formation fracturing treatment may beresumed or the well placed on production again.

The breakdown of the formation to be fractured is normally indicated atthe earths surface by a'rather sudden drop in the pressure required toinject the carrying agent into the formation. The start of a screen-out,however, is usually indicated by a pressure build-up after the formationbreakdown occurs and as the slurry of carrying agent and particulatedmaterials is being injected into the formation being fractured.

AFollowing the breakdown of the formation 16 to be fractured, a slurryor blend of carrying agent and particulated materials 64 is pumpedthrough the perforations 30 and into the fractures 66 to expand andextend the fractures 66 and to carry particulated materials 64 into andthrough the fractures 66 to act as propping agents to holdthe fractures66 open when the fracturing treatment is finished and hydrostaticpressure is released at the well head (permitting the over-burden tosettle) and the fractures tend to close.

Occasionally the concentration of particulated material per unitvolumeof carrying-agent exceeds the amount which can, -under particular wellconditions, be carried into the formation by the carrying agent and theparticulated material tends to screen-out on the face of the formationadjacent to the perforations 30. F)Chis screening out of theparticulated material causes an increase in the pressure required toinject fluid into the formation 16, as previously mentioned. Often thepressure required to inject fluid (during screen-out) exceeds thepressure which the well lor treating devices can withstand, and therebyfurther injection of particulated material is prevented.

When the start of a screen-out is indicated (by a rise in hydrostaticpressure indicated at the earths surface 1'2), clear carrying agent ispumped or pumped at a more rapid rate) through the tubing 28 to quicklydilute the slurry or blend of particulated material and carrying agent,thereby to increase lthe volume of carrying agent per unit ofparticulated materials as the slurry is pumped into the formation,carrying the former excess of particulated materials 64 into theformation 16 before a complete screen-out occurs. The carrying agent 68pumped through the tubing 26 also often helps to clear up a partialscreen-out by its washing action as it ows from the tubing to theformation.

Changing the ratio of the amount of particulated materia-ls to theamount of carrying agent at the blender prior to injecting the mixtureinto the annulus can seldom stop a screen-out from occurring once thepressure buildup is noted because of the time lag in getting the moredilute blend to the perforations from the surface.

When a pressure buildup indicating the start of a screenout occurs,usually the ratio of particulated materials to carrying agent (or otherfavorable change in the consistency of the slurry) is also reduced atthe blender 46 as well as the pumping of clear carrying agent or diluentthrough the tubing 26 to immediately lower, at the face of the formationbeing fractured, the ratio of particulated materials to the Huidcarrying the particles into the formation 16. Thus, when the weakerslurry injected at the surface 12 does reach the formation 16 the rateof pumping of clear carrying agent through the tubing may be decreasedor even stopped until the possible occurrence of another indication of ascreen-out. The remainder of the fracturing treatment, following thestep or steps (in event of a recurrence of the symptoms indicating ascreen-out is about to occur) to prevent a screenout, is conventional.rflhat is, a predetermined amount of particulated materials blended intocarrying agent is injected into the formation, usually followed by aninjection of clear carrying agent (which need not be the same carryingagent as that used in the slurry) through the annulus in an amountsuliicient to drive into the formation the slurry of particulatedmaterials and carrying agent which are in the annulus.

The term diluent is understood to mean any liquid capable of dilutingthe slurry or blend of particulated material and carrying agent. Suchliquid may or may not be miscible with the carrying agent and need notbe completely free from particulated material. It may be, for example, amore dilute slurr/ than that being injected vas part of the conventionalfracturi-ng procedure.

While lease oil has been heretofore mentioned as a carrying agent, manyother fluid materials may be used .as carrying agents or diluents.Thickened water, thickened acid, water, acid, emulsions of oil and wateror acid, thickened or emulsied crude oil or refined oil are othercommonly used carrying agents. Demulsifying or other additive agents mayor may not be used in connection with the carrying agents or diluents,depending on the conditions under which a particular well treatmentismade.

Any conventional propping agent such as sand, particulated walnutshells, etc. may be used in connection with this invention. Ottawa sandis a preferred sand. The mesh size of the particulated material may bethe same as for conventional fracturing treatments,

It should be emphasized that this .invention is equally applicable totreatments in which sand and oil (that is, a slurry or blend ofparticulated materials and carrying agent) are initially pumped into theannulus without any prior injection of clear carrying agent or otherliquid to make the initial formation breakdown.

Ina typical fracturing operation in accordance with the teachings ofthis invention, 22,000 pounds of sand (20,000 pounds 20/40 mesh and2,000 pounds 40/60 mesh sand) were blended with about 14,700 gallons oflease oil. The average sand concentration was thus about 1.5 pounds pergallon of carrying agent. The sand and oil slurry were yinjected in -theannulus between the 7 .inch (nominal Vouter diameter) well casing andthe 2 inch (nominal outer diameter) well tubing. Fourteen minutes afterthe start of the sand-oil injection, the slurry of 14,700 gallons of.oil and the sand had been pumped into the casing annulus. Pumping ofstraight crude oil was then pumped down the annulus (the pumping of thesand-.oilslurry and the crude oil was a continuous operation.) to Hushthe sand-oil `slurry into the formation 16 to be fractured. After 6minutes of pumping of crude oil, a pressure rise in the annulus feedingline 40 indicated a screen out was forming. Lease oil was then pumpeddown the `tubing 26 to dilute the concentration of sand 64 Ebeingflushed into the formation 16 from the annulus. A drop .in injectionpressure on the annulus line 40 indicated the screen .out had beenavoided. About 16,800 gallons of crude oil were pumped down the annulusand the tubing. The well was 6,240 feet deep. No record was kept showingthe proportion of the crude oil to the total crude oil pumped which waspumped down the tubing to prevent the screen-out. However, 6 pumperunits were used on the treatment, one pumper being ,connected to injectoil into the tubing 26. Because of `friction losses, differences inpumper eiciency and similar varying parameters no exact ow ratio, at theperforations 30, may be made between the tubing flow and annulus ow.

As mentioned previously, the invention may also be :practiced by pumpingthe slurry through the tubing 26 and clear carrying agent through theannulus.

I claim:

1. The method of preventing screen-outs during the injection into anearth formation penetrated by a well bore -having casing and tubingdisposed concentrically therein of a slurry of particulated material ina fluid carrying agent, comprising, when a rise in injection pressureoccurs which indicates the beginning of a screen out, pumping a diluentwhich is free from particulated material into vsaid rwell bore at alocation which is directly adjacent to the point of entry of said slurryinto said formation whereby to dilute the amount of particulatedmaterial per unit volume of carrying agent entering said formation.

2. The method according to claim 1, wherein said particulated materialis sand. Y

3. The method according to claim 1, wherein said carrying agentcomprises an oil. f

4.- 'Ihe method according to claim 1, wherein said carrying agent of theslurry is a Huid different from that of the diluent.

5. The method according to claim l, wherein said slurry is injectedthrough the annulus between said tubing and :asing and said diluentwhich is free of particulated material is pumped through said tubing.

6. The method according to claim 1, wherein said slurry is injectedthrough said tubing and said diluent which is tree of particulatedmaterials is injected through the annulus between said tubing andcasing.

7. The method of preventing screen-outs during the iniection into anearth formation penetrated by a Well bore of a slurry of particulatedmaterial and fluid carrying agent, comprising, when the beginning of ascreen-out is lindicated, injecting simultaneously with the injection ofsaid slurry additional tluid carrying agent having less particulatedmaterial blended therein than said slurry into said well bore at alocation adjacent to the point of entry of said slurry into saidformation whereby to dilute the amount of particulated material per unitvolume of carrying agent entering said formation.

8. The method according to claim 7, wherein the additional carryingagent injected into said Well bore adjacent to the point of entry ofsaid slurry into said formation has only a trace of particulatedmaterial blended therein.

9. The method of preventing screen-outs during the injection into anearth formation penetrated by a well bore of a slurry of particulatedmaterial and fluid carrying agent, comprising, when the beginning of ascreenout is indicated, comprising pumping a diluent into said well boredirectly adjacent to the point of entry of said slurry into saidformation whereby to dilute the amount of particulated material per unitvolume of iluid. entering said formation.

10. The method according to claim 9, wherein said diluent and saidcarrying agent are miscible.

11. The method according to claim 9, wherein said diluent and saidcarrying agent are immiscible.

References Cited in the le of this patent UNITED STATES PATENTSBenckenstein July 25, 1944 2,815,815 Hower et a1 Dec. 11D, 1957

1. THE METHOD OF PREVENTING SCREEN-OUTS DURING THE INJECTION INTO ANEARTH FORMATION PENETRATED BY A WELL BORE HAVING CASING AND TUBINGDISPOSED CONCENTRICALLY THEREIN OF A SLURRY OF PARTICULATED MATERIAL INA FLUID CARRYING AGENT, COMPRISING, WHEN RISE IN INJECTION PRESSUREOCCURS WHICH INDICATES THE BEGINNING OF A SCREEN OUT, PUMPING A DILUENTWHICH IS FREE FROM PARTICULATED MATERIAL INTO SAID WELL BORE AT ALOCATION WHICH IS DIRECTLY ADJACENT TO THE POINT OF ENTRY OF SAID SLURRYINTO SAID FORMATION WHEREBY DILUTE THE AMOUNT OF PARTICULATED MATERIALPER UNIT VOLUME OF CARRYING AGENT ENTERING SAID FORMATION.